runtime: fix return type for bsdthread_register in comments

runtime: fix return type for bsdthread_register in comments

The return type for bsdthread_register is int32. See
runtime/os_darwin.go.

This change also rewrites declaration comments for go functions to
use go syntax and fixes vet errors in sys_darwin_amd64.s.

Change-Id: I7482105f7562929e0ede30099efac9e76babd8a3
Reviewed-on: https://go-review.googlesource.com/3260
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>